Difficult question to answer because you are only one person, but i want you to think about it, look on the web at popularity and so forth and tell me, What would fans want for x-men 4? or any sequel/prequel to be honest...

Hopefully this will give us a better idea as to why the studio does what it does.

My theory is, there has always been that devide between humans and mutants, in x1 to x3 the tension lead to a war, the end result left several mutants de-powered, and several humans dead... The Legacy Virus and the Rise Of Apocalpyse would give the two species a common enemy...

Think Watchmen, to unite the world they had to find a common enemy... which was supposed to be a fake alien threat, but in the movie they changed it to Dr.Manhatten.

Apocalpyse wants a survival of the fitest world... they dont necessary have to be mutants but it sure does give them an advantage.

Legacy Virus, threat of sentinals being created, horsemen, apocalpyse.... to unite the world they need a common enemy would be fitting to end the second triology on a good note.